Clinical Vignette 67

Probably the most beautiful parasite I have seen under the microscope.

View from the microscope via two photographs taken through the eyepiece.

Question: What is the name of the parasite and what disease does it cause in humans?

[Updated 28 September 2016]

This rare (in humans) parasite is Dicroceolium dendriticum – the lancet liver fluke. Humans are only accidental definitive hosts, while ruminants such as sheep, goats and cattle are the main definitive hosts. It is not found in Southeast Asia – I saw this specimen under a microscope whilst participating in the Gorgas course in Peru.

Its life cycle is quite a marvel – eggs which are passed out in feces are consumed by snails, and the larva migrate to the snails’ guts, where a cyst is formed around them by the snails trying to protect themselves. The cysts are then passed out, and consumed by ants (who apparently consume snails’ slime for moisture). One cyst contains hundreds of “baby” flukes, which break out, migrate and encyst themselves again within the ant as metacercaria.

Now the creepy part – one of ther metacercaria will migrate to the ant’s central nervous system and take control of the ant, such that it will climb to the top of a blade of grass in the evenings and clamp it’s jaws on the grass, staying there until dawn when it returns to the ant hive again (so that it – along with its fluke infestation – doesn’t die due to the sun’s heat). This repeated action maximizes the chance that a definitive grazing host will eat the infected ant, whereupon the flukes will migrate to the liver and finally mature into their adult stage (as seen under the microscope), laying hundreds of thousands of eggs in their lifetime to continue their perilous existence.

So humans can only get infected if they somehow consume an (uncooked) infected ant.

The clinical presentation is otherwise largely similar to other human liver flukes – mostly asymptomatic, occasional biliary colic, very rarely liver cirrhosis and eventually cancer.